1. Field of the Invention
This invention relates generally to windshield wipers and in particular to windshield wipers having a silicon rubber wiper blade that incorporates PTFE.
2. Description of Related Art
Rain, sleet, and snow have always presented a vision problem for the driver of a moving vehicle. The windshield wiper blade has attempted to minimize the problem by clearing the windshield of the light obstructing moisture and debris. Such blades are typically formed of rubber or rubber-like materials. Over the years, wiper blades have been modified in many ways in order to enhance wipe quality and therefore visibility during precipitation. In some instances, the configuration of the blade has been changed to give a plurality of contact surfaces on the blade. Various modifications have been introduced to improve the consistency and integrity of the wiping edge.
Wiper designers have developed silicone-rubber-based wiper blades with some success. Silicone rubber is a superior material to natural rubber for several reasons. Silicone rubber, i.e., high molecular weight, vulcanizable polydiorganosiloxane, is able to withstand wide temperature variation without an appreciable effect on its physical properties. Further, silicone rubber is virtually unaffected by ultraviolet radiation, even over long periods of time. It is also resistant to ozone, oil, salt, water and other road and automotive chemicals.
Silicone rubber as used for wiper compositions has had one significant drawback: it has an unacceptably high coefficient of friction with respect to glass. Some of the early silicone wiper blades exhibited such a high coefficient of friction that the wiper blades could tear loose from the wiper frame when wiping the windshield. Less catastrophic effects of this high coefficient of friction include an unacceptably loud squeak or chatter as the wiper traverses the windshield, and unacceptably high loads on the windshield wiper motor. The silicone wiper blades produced today have improved significantly but wiper designers continually search for improved solutions that would reduce the friction between the wiper blade and the windshield.
Polytetrafluoroethylene (PTFE) has been used in conjunction with wiper blades in an attempt to decrease friction between the wiper blade and the windshield. However, the wiper blades are typically coated with PTFE after the blade is cured. Coating a cured blade with PTFE is less than desirable because the PTFE will wear off over time, thereby reducing the improved frictional characteristics of the wiper blade.
Japanese Patent Application No. Hei 5[1993]-117530, by Hiroshi Honma, (the “Honma Application”) describes compounding a fluoro resin powder from 0-10 parts by weight with a silicone rubber formulation for wiper blades. The application teaches that the formulation provides excellent climate resistance and causes no vibration or squeaking. Fluoro resin powder, such as PTFE, is added to the compound in a preferable amount of 1-10 parts by weight, and an average particle size of 40:m. As described in more detail below, the primary problem with compounding PTFE as described in the Honma Application is that the particle size of the PTFE hinders the manufacturability of the compound. Larger particle sizes of PTFE tend to increase the plasticity of the silicone rubber compounds, which reduces the ability to extrude the compound, and in some cases the ability to mold the compound.
A need therefore exists for a windshield wiper blade made of a silicone rubber compound that provides excellent friction characteristics when wiping a windshield. The reduced friction characteristics of the wiper blade will preferably allow a significant reduction in the force required to move the wiper blade across the windshield and will reduce the amount of chatter, squeaking, jumping, and other noise inducing and performance reducing actions associated with current wiper blades. A need further exists for a windshield wiper blade having these properties that is simple and inexpensive to manufacture. Preferably, the materials used in the wiper blade compound will be readily available and inexpensive. Finally, a need exists for a wiper blade compound that has a relatively low plasticity, thereby allowing the compound to be easily formed by a variety of manufacturing methods, including extrusion.